1. Field of the Invention
This invention relates to an optical head device for carrying out recording or reproducing of data by irradiating a recording face of an optical disk with a laser beam, and more particularly to a technique usefully applied to a laser beam detecting mechanism which intends to adjust the output of the laser beam.
2. Description of the Related Art
For example, in an optical head device for carrying out recording/reproducing for a CD (compact disk), DVD (digital versatile disk), an optical disk for a blue-violet laser, etc., it has been conventionally known to detect the output strength of the laser beam to adjust the output of the laser beam by feedback control.
Traditionally, as previously known methods for detecting the output strength of the laser beam, there were a method of detecting a part of the laser beam branched by a prism or the like from the laser beam traveling to an optical disk, a method of detecting the laser beam spread outward from the laser beam emitted from a semiconductor laser and received by an optical sensor, and a method of detecting the laser beam thus spread outward and reflected by a reflecting plate using the optical sensor (for example, JP-A-56-7250, JP-A-2001-84635 and JP-A-2002-319699).
However, when the output strength of the laser beam is detected in such a manner that a part of the laser beam branched by the prism or the like is detected, power loss of the laser beam to be employed for read/write of data occurs disadvantageously. The power loss of the laser beam is a serious problem because a high laser output is required for data recording, particularly for improvement of the recording speed.
The method of detecting the laser beam spread outward from the center in order to detecting the output strength of the laser beam presents a problem that it is difficult to acquire a quantity of light necessary for detection. Since the laser beam has high directivity, its quantity of light extremely decreases from the center toward the outside. Therefore, in order to directly detect the beam deviated to the outside by an optical sensor, an optical sensor must be arranged at the position in the close proximity of an opening of a collimator. If the optical sensor is deviated outward from that position, the detected quantity of light extremely decreases. As a result, it is difficult to mount the optical sensor appropriately.
In order to prevent reduction in the quantity of light, if the optical sensor provided with a condenser lens in front of an optical element is employed, inversely, because of hindrance by the condenser lens, the sensor element cannot be arranged in the close proximity of the opening of the collimator. As a result, the quantity of light which reaches the sensor element is reduced.
Where the beam deviated from the center to the outside is reflected by a reflecting mirror (reflecting plate) so that the reflected beam is detected, if any means is not provided as in JP-A-56-7250, the weak light deviated from the center of the laser beam is further scattered to reach the optical senor. Thus, a required quantity of light cannot be obtained. Otherwise, the reflecting plate must be arranged near to the center of the laser beam. In this case, the power loss of the laser beam occurs as in the case of branching of the laser beam by the prism.
Further, as disclosed in JP-A-2001-84635, if the reflected light is focused at the position of the sensor element of the optical sensor through a lens, the reflecting mirror and optical sensor must be arranged with high accuracy in their attaching position and angle. This leads to a problem of an increase in assembling cost.
JP-A-2002-319699 discloses that the reflecting mirror is formed in a concave shape like the inside of an elliptical globe so that the reflected light is focused at the position of the sensor element of the optical sensor. However, such a configuration makes it difficult to mold the reflecting mirror appropriately. This leads to problems of increasing component cost, and of requiring high accuracy in the attaching position and angle.